The invention relates to methods and apparatus for converting optical pulses generated by compact, low-intensity long-pulse pump sources, such as diode, fiber or solid-state lasers, into high-energy ultrashort optical pulses through the use of optical parametric amplifying media, and to particular applications of the same.
As used herein, the term "high-energy pulses" refers to optical pulses having energy levels higher than that obtainable directly from ultrashort-pulse oscillators. Typically, compact mode-locked oscillators produce pulses with maximum energies at the 10 nJ level. Therefore, pulses with energies of more than 10 nJ are defined herein as high-energy pulses.
Ultrashort pulse lasers and amplifiers belong to a particular class of laser devices which generate ultimately short optical pulses (at the optical-wavelength limit) with durations in the femtosecond (10.sup.-15 s) to picosecond (10.sup.-12 s) regimes. The applications of such pulses are determined by their characteristic features, which include short duration, high peak power and high spatial and temporal coherence. As described in more detail below, advantageous use can be made of such pulses in fields such as machining, medicine (surgical applications including tissue ablation, tissue removal, precise incisions, sclera and skin surgery, intraocular surgery and molecular surgery), LIDAR, scientific measurement and imaging.
Diode lasers are compact sources of laser emission which possess two unique technological advantages. First, diode lasers provide direct conversion from electrical to optical power with high efficiency. Second, they are monolithic devices with small dimensions (typically less than 1 mm). Consequently, their parameters such as size, robustness, reliability, life-time, manufacturability and cost are substantially better than corresponding parameters of other laser structures, such as gas, dye or bulk solid-state lasers. These key features make them ideally suitable for developing commercially viable laser sources. However, direct use of diode lasers in the generation of high-energy ultrashort pulses is limited. Essentially this is determined by the small cross-sectional area of a single-mode diode. Catastrophic damage to the diode and severe nonlinear distortions of the ultrashort pulses restricts obtainable peak intensities. Additionally, due to the same small cross-sectional area, stored energy and saturation fluency are also limited. Maximum energies directly obtainable from a laser diode are limited to about 100 pJ, which is at the lower limit of practically significant ultrashort pulse energies. While the effective cross-sectional area of a laser diode can be increased by resorting to multiple-transversal-mode structures or multiple-stripe structures, the requirement of spatial and temporal coherence does not permit direct generation of ultrashort pulses with such devices.
This necessitates using diodes as pump sources for other classes of ultrashort-pulse lasers and amplifiers in order to develop practical systems. Rare-earth doped fiber lasers represent one such class of devices and are closest to semiconductor gain media in compactness, as mainly determined by the small transverse dimensions of the fiber. The typical diameter of a fiber structure is less than 1 mm. Unlike a semiconductor laser, a fiber laser can have a length of several meters, but due to the small transverse dimensions it can be spooled to occupy a small space. In effect, the fiber laser is a one dimensional structure, with the transverse distribution of the optical field being the same at any longitudinal position. Rare-earth doped fibers can be diode-laser pumped. For example, known Er-doped fiber laser systems have been pumped with existing high-power laser diodes emitting at 1480 nm or 980 nm.
As reported in Broad-area Diode-pumped 1 W Femtosecond Fiber System, a. Galvanauskas, M. E. Fermann, D. Harter, J. D. Minelly, G. G. Vienne, J. e. Caplen, Conference on Lasers and Electro-Optics, vol. 9 1996 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1996) pp. 495, hereby incorporated herein by reference, high power multimode diode pump light is efficiently converted into a high power ultrashort pulse output by fiber cladding-pumping techniques and chirped pulse amplification. In general, chirped pulse amplification is necessary for any quantum amplifier in order to extract the maximum available energies without incurring nonlinear distortion of the ultrashort pulses or optical damage to optical components or the gain medium. Typically, the peak intensity of an ultrashort pulse, with an energy equal to the saturation energy, is higher than the saturation fluency of the medium.
However, in order to preserve spatial and temporal coherence and to sustain ultrashort pulses, the fiber output has to be single-mode. This puts constraints on the fiber core size and, consequently, on the maximum obtainable pulse energies and peak intensities, for reasons here equivalent to the case of a single-mode semiconductor laser. Maximum obtainable energies for a single-mode fiber, however, are substantially higher than for a semiconductor. The maximum, saturation-fluency-limited energies have already been experimentally produced with some diode pumped Er-fiber chirped pulse amplification systems, yielding pulse energies of more than 10 .mu.J after amplification and recompression. However, for a variety of practical applications, such as micromachining, optical surgery, etc. much higher ultrashort pulse energies are required (typically in the range of 1 to 10 mJ). To obtain these pulse energies, bulk quantum amplifiers have been conventionally used. In a bulk medium, the beam size is substantially larger than the single-mode guided beam in a fiber or a semiconductor structure, alleviating the problem of high peak intensities. Furthermore, certain solid-state gain media have properties which permit design of compact devices. However, a number of limitations, as determined by the general properties of quantum amplifiers, make it practically difficult to implement compact solid-state designs for direct amplification of ultrashort high-energy pulses. This is revealed by considering the general properties of a quantum amplifier.
A quantum amplifier stores pump energy in an upper level of an optical transition state, which can be harvested by a passing signal through the action of optical stimulated emission. Known solid-state ultrashort-pulse amplifying arrangements include single or multiple-pass energies in the 1 .mu.J to 1 J range. Chirped pulse amplification is a necessity for these systems.
However, bulk lasers and amplifiers have notable limitations. First, solid-state lasers and amplifiers are substantially larger and more expensive than their semiconductor and fiber counterparts. The size and cost are mainly driven by the cumbersome pump sources required, e.g., high-power Ar lasers or lamps. Diode pumping is possible for few such systems. It is necessary to pump a quantum amplifier within the fixed absorption band of the particular gain medium. For many media, this eliminates or restricts diode-laser pumping, because reliable and high-power pump diodes are currently available at only a few wavelengths. For example, the most popular solid-state medium for ultrashort pulse generation is Ti-sapphire, which can not be directly diode laser pumped.
Second, quantum amplifiers have a limited gain bandwidth, which is determined by the width of the optical transition in the particular gain medium. The narrow width of the gain bandwidth substantially limits the use of certain materials for amplifying ultrashort pules.
Third, intrinsic properties of the gain medium, such as the lifetime of the excited optical transition and the simulated emission cross-section, set limits on the maximum extractable average power and pulse energy from a particular quantum amplifier.
Fourth, at high power levels, bulk amplifiers are susceptible to thermal effects which change the optical properties of the gain medium. This makes the operation of such devices sensitive to changes in the environment.
An alternative approach for achieving optical amplification is to employ optical parametric amplification (OPA) in a nonlinear material. According to the OPA approach, pump energy is not stored in the material but directly transferred from the pump into the signal; the nonlinear material only mediates the process. Pulse distortions through phase distortion can in general be avoided because the second-order nonlinearity is much stronger than the third order (responsible for self or cross-phase modulation). The maximum obtainable energy is essentially limited by the damage threshold of the particular material. The required pump wavelength and the available amplification bandwidth are determined by the fundamental optical properties of the particular crystal, such as orientation and size of the refractive index ellipsoids at the interacting wavelengths in conventional birefringence phase-matching. These fundamental optical properties also determine the useful crystal orientation and, consequently, the magnitude of the nonlinearities which can be utilized. In practice, this puts limitations on the pump wavelengths and bandwidths accessible with the available nonlinear materials and, in general, leads to the high energies required to pump such amplifiers. As a result of the above limitations, parametric interaction at present is generally used as a means of converting the wavelength of an optical signal, not as a means of energy amplification.
In Powerful femtosecond Pulse Generating Chirped and Stretched Pulse Parametric Amplification in BBO Crystals, A. Dubietis, G. Jonusauskas, and A. Piskarskas, Opt. Comm. 88, 437 (1992), hereby incorporated herein by reference, it is suggested that high-energy ultrashort optical pulses may be obtained through the use of optical parametric amplifiers instead of conventional quantum amplifiers. The article teaches that ultrashort optical pulses must be stretched to match the duration of the pump pulse for efficient energy transfer from the pump into the signal. This work demonstrated 1:30 conversion from a 3 mJ pump at 0.53 .mu.m into a 100 .mu.J signal at 1.06 .mu.m with short (about 5 ps) stretched pump pulses.
However, the work of Dubietis et al. does not teach energy conversion from low to high brightness beams, nor how to achieve a compact source of high-energy ultrashort pulses through the use of compact pump sources, such as diode, fiber or microchip lasers. (One of the problems that would be encountered is that to demonstrate the same conversion efficiency with longer pump pulses (in the nanosecond range), pulse energies would have to be increased proportionally by a factor of about 100 (into the Joule range). At present it is difficult to get such high energies from compact pulse sources.) Also, this work does not remove the limitations on the pump wavelength and the gain bandwidth of an ultrashort-pulse amplifier. Additionally, in this work, both the pump and amplified pulses were from the same laser source. No method of synchronizing long-pulse pump and short pulse sources is suggested. It is problematic to synchronize pulses from a conventional Q-switched pump laser with ultrashort pulses from a mode-locked source.
Although the foregoing primarily stresses the use of diode lasers as pump sources, it is axiomatic that the pump source may be formed of a combination of a diode laser and one or more serially arranged laser sources at least the first stage of which is capable of being diode-laser pumped. For example, the pump source may be constituted of a diode laser which pumps a rare-earth fiber laser or a Q-switched pulse source.